1. Field of the Invention
The present invention relates to a variable capacitor which can operate satisfactorily in a high-frequency region with a low dielectric loss, as well as to a circuit module and a communications apparatus. More particularly, the invention relates to an easy-to-handle variable capacitor that has the advantages of allowing great capacitive variation through application of a voltage; minimizing nonlinear distortion which is caused by a high-frequency signal; offering sufficiently high electrical strength; and functioning properly regardless of the polarity of a voltage to be applied, as well as to a circuit module and a communications apparatus that employ said variable capacitor.
2. Description of the Related Art
Conventionally, a silicon oxide (SiO2) thin film, a silicon nitride (Si3N4) thin film, and a tantalum oxide (Ta2O5) thin film have commonly been used for forming an IC-adaptable dielectric thin film capacitor. As compared with these thin films, a strontium titanate (SrTiO3: paraelectric substance) thin film and a barium strontium titanate ((Ba,Sr)TiO3: ferroelectric substance) thin film each exhibit a higher dielectric constant, and thus show promise nowadays for forming an IC-adaptable dielectric thin film capacitor that is smaller in area than ever.
There have been proposed to date thin-film capacitors that employ, as their dielectric layers, a ferroelectric oxide thin film having a perovskite structure such as strontium titanate or barium strontium titanate (for example, refer to Japanese Unexamined Patent Publication JP-A 11-260667 (1999)).
FIG. 18 is a sectional view showing a thin-film capacitor disclosed in JP-A 11-260667. The thin-film capacitor 200 is constructed by forming, on a supporting substrate 201, a first electrode layer 202, a thin-film dielectric layer 203, and a second electrode layer 204 one after another in the order named by deposition. More specifically, on the supporting substrate 201, a conductor layer to be formed into the first electrode layer 202 is formed by deposition over substantially the entire surface thereof. Then, patterning is performed to impart a desired electrode configuration to the conductor layer, thereby realizing the first electrode layer 202. Next, on the first electrode layer 202 is formed the thin-film dielectric layer 203. The thin-film dielectric layer 203 is formed, by the thin-film forming technique, with a mask placed at a predetermined position, or formed by a spin coating, followed by performing patterning thereon to obtain a desired configuration. The thin-film dielectric layer 203 is then subjected to a heat-hardening treatment on an as needed basis. After that, a conductor layer to be formed into the second electrode layer 204 is formed over substantially the entire surface of the thin-film dielectric layer 203. Then, patterning is performed to impart a desired electrode configuration to the conductor layer, thereby realizing the second electrode layer 204. In this way, the thin-film capacitor 200 is fabricated. Note that, in this construction, the facing region of the thin-film dielectric layer 203 that is actually held between the first electrode layer 202 and the second electrode layer 204 constitutes a capacitance producing region.
In putting the thin-film capacitor 200 into practical use, by adjusting the dielectric constant of the thin-film dielectric layer 203 to be a desired value through application of a predetermined direct current bias voltage (bias signal), it is possible to control the capacitance characteristic appropriately. As a result, the thin-film capacitor 200 is able to function as a variable capacitor normally. Note that, in the thin-film capacitor 200, the first electrode layer 202 and the second electrode layer 204 play two individual roles: an electrode for generating a bias signal-controlled capacitance component of predetermined level and an electrode for feeding a bias signal to the thin-film dielectric layer 203.
In the thin-film capacitor 200 of conventional design, however, as shown in the equivalent circuit diagrams depicted in FIGS. 19A and 19B for example, a bias signal is supplied through an external circuit (bias supply circuit) G disposed on a wiring board on which the thin-film capacitor 200 is mounted.
In FIG. 19A, reference symbol A indicates a point of connection of the thin-film capacitor 200 and the bias supply circuit G. Arranged between the point of connection A and a bias terminal V is a choke coil 205 acting as an inductance component.
On the other hand, in FIG. 19B, in the bias supply circuit G is formed a strip line 206 having a line length of λ/4, where λ indicates the wavelength of a high-frequency signal at which the thin-film capacitor 200 is operated. The strip line 206 has its bias terminal V-side end which is connected to ground. Moreover, between the bias terminal V-side end of the strip line 206 and the ground portion is formed a direct current regulating capacitance element 208.
Because of the above-described configuration, the thin-film capacitor 200, namely the conventional variable capacitor poses the following problems. In order to put the thin-film capacitor 200 into practical use, not only the thin-film capacitor 200 in itself, but also the bias supply circuit G appropriate to the structure and property of the thin-film capacitor 200 needs to be prepared for use. The bias supply circuit G is disposed on the wiring board. It is thus necessary to design the bias supply circuit G in conformity with the thin-film capacitor 200 to be mounted on the wiring board. In this case, very complicated and time-consuming process is required to make adjustment to the bias supply circuit G. Furthermore, the thin-film capacitor 200 and the bias supply circuit G are fabricated independently of each other. This leads to an undesirable increase in the size of the entire construction.
As another problem, in the conventional thin-film capacitor 200, the high-frequency signal terminal and the bias terminal V are used in an interchangeable manner. It is thus necessary to separate a high-frequency component (signal component of a high-frequency signal) and a direct current component (bias signal) from each other by using an additional component such as the choke coil 205 in an external circuit.
As still another problem, the capacitance of the thin-film capacitor 200 cannot be caused to vary greatly without making the thin-film dielectric layer 203 smaller in thickness. However, the capacitance of the capacitor is proportional to the area of the dielectric substance, and yet is inversely proportional to the thickness thereof. Therefore, in a case where the thin-film dielectric layer 203 is designed to have a small thickness, the lowering of the capacitance cannot be achieved without reducing the mutually facing areas of the first electrode layer 202 and the second electrode layer 204. This makes the manufacturing process difficult.
As further another problem, in the case of using the thin-film capacitor 200 as a high-frequency electronic component, both a direct current bias voltage for bringing about capacitive variation and a high-frequency signal voltage (high-frequency voltage) are applied to the thin-film capacitor 200 at the same time. In this case, however, when a high-frequency voltage of unduly high level is applied, the capacitance of the thin-film capacitor 200 is caused to vary not only with the application of the direct current bias voltage but also with the application of the high-frequency voltage. This is liable to result in occurrence of distortion such as waveform distortion or intermodulation distortion in the high-frequency electronic component. The waveform or intermodulation distortion cannot be minimized without decreasing the high-frequency voltage-induced capacitive variation by lowering the intensity of the high-frequency electric field. In order to decrease the capacitive variation, it is effective to make the thin-film dielectric layer 203 larger in thickness. However, as the thickness of the thin-film dielectric layer 203 is increased, the direct current electric field intensity is decreased correspondingly, which results in an undesirable decrease in the degree of the capacitive variation for which the direct current bias voltage is responsible.
A problem also exists in power handling capability. Since an electric current is allowed to pass through the capacitor easily in a high-frequency region, it follows that heat is generated in the capacitor due to the loss resistance that the capacitor sustains when operated in a high-frequency region. This gives rise to the risk of capacitor breakdown. In order to overcome such a problem associated with power handling capability, it is also effective to increase the thickness of the thin-film dielectric layer 203 so as to reduce the extent of heat generation per unit volume. However, as described above, as the thickness of the thin-film dielectric layer 203 is increased, the direct current electric field intensity is decreased correspondingly, which results in an undesirable decrease in the degree of capacitive variation for which the direct current bias voltage is responsible.
As further another problem, in the thin-film capacitor 200, in general, interfacial condition between the thin-film dielectric layer 203 and the first electrode layer 202 differs from that between the thin-film dielectric layer 203 and the second electrode layer 204. Therefore, in the case of applying a direct current bias voltage, there could be variation in leakage current characteristic depending upon the polarity of the direct current bias voltage applied. Such a variation has been well known as a so-called Schottky emission current phenomenon. More specifically, in a case where the first electrode layer 202 and the second electrode layer 204 are made of different materials, the work function of the first electrode layer 202 with respect to the thin-film dielectric layer 203 differs from that of the second electrode layer 204 with respect to the thin-film dielectric layer 203. In this case, when a leakage current is generated through the emission of electrons, the magnitude of the leakage current differs depending upon whether the electrons are emitted from the first electrode layer 202 or emitted from the second electrode layer 204. That is, the leakage current varies according to the polarity of the direct current bias voltage applied. On the other hand, even if the first electrode layer 202 and the second electrode layer 204 are made of the same material, interfacial condition between the first electrode layer 202 and the thin-film dielectric layer 203 formed thereon microscopically differs from that between the thin-film dielectric layer 203 and the second electrode layer 204 formed thereon, in consequence whereof there results a difference in work function. Also in this case, the leakage current varies according to the polarity of the direct current bias voltage applied. Accordingly, in order for the thin-film capacitor 200 to operate properly, it is necessary to take into account the polarity of the direct current bias voltage applied not only in a designing stage but also in a mounting stage.